Nitrided titanium alloys

ABSTRACT

A NOVEL GROUP OF NITRIDED ALLOYS HAVING EXCELLENT WEAR AND ABRASION RESISTANCE AND UTILITY AS CUTTING TOOL MATERIALS CONTAINING (A) TITANIUM, (B) ONE OR MORE METALS OR THE GROUP VANADIUM, COLUMBIUM, OR TANTALUM, AND (C) MOLYBDNUM OR TUNGSTEN. THE ALLOYS CAN BE READILY FABRICATED TO SHAPE AND THEN HARDENED BY NITRIDING TO PRODUCE HIGH SURFACE HARDNESS.

United States Patent 3,677,832 NITRIDED TITANIUM ALLOYS Ray J. Van Thyne, Oak Lawn, and John J. Rausch,

Antioch, 111., assignors to Surface Technology Corporation, Stone Park, 111. No Drawing. Filed Mar. 4, 1970, Ser. No. 16,572 Int. Cl. C22c 27/00; C23c 11/14 U.S. Cl. 148-315 7 Claims ABSTRACT OF THE DISCLOSURE A novel group of nitrided alloys having excellent Wear and abrasion resistance and utility as cutting tool materials containing (a) titanium, (b) one or more metals of the group vanadium, columbium, or tantalum, and (c) molybdenum or tungsten. The alloys can be readily fabricated to shape and then hardened by nitriding to produce high surface hardness.

BACKGROUND OF THE INVENTION This invention relates to a novel group of ternary or higher alloyed metals which consist essentially of:

(a) titanium;

(b) at least one metal selected from the group consisting of vanadium, columbium and tantalum; and

(c) at least one metal of the group consisting of molybdenum or tungsten.

These alloys can be nitrided and when so nitrided demonstrate high surface hardness without accompanying brittleness and may be used in applications requiring wear and abrasion resistance.

It is well known that titanium can be nitrided to form a hard surface layer thereon, but such material shows a chipping propensity. The nitriding of titanium-rich alloys, i.e., containing about 90% titanium has been studied previously (for example, see E. Mitchell and P. I. Brotherton, J. Institute of Metals, vol. 93 (1964), p. 381). Others have investigated the nitriding of hafnium-base alloys (F. Holtz et al., U.S. Air Force Report IR-7l8-7 (II), 1967); molybdenum alloys (U.S. Pat. No. 3,161,949); and tungsten alloys (D. I. Iden and L. Himmel, Acta Met, vol. 17 (1969), p. 1483). The treatment of tantalum or columbium and certain unspecified tantalum base alloys with air or nitrogen or oxygen is disclosed in US. Pat. 2,170,844 and the nitriding of columbium is discussed in the paper by R. P. Elliott and S. Komjathy, AIME Metallurgical Society Conference, vol. 10, 1961, p. 367.

In our copending patent applications Wear Resistant Materials Ser. No. 755,658 now U.S. Pat. No. 3,549,427 and Wear and Abrasion Resistant Materials Ser. No. 755,662, now U.S. Pat. No. 3,549,429, we have disclosed and claimed certain nitride three through seven metal alloy systems which are characterized by excellent cutting performance. Counterparts to said U.S. applications have now been issued as Belgium Pats. 720,398 and 720,399. Such applications and Belgium patents are directed to nitrided alloys containing Patented July 18, 1972 (a) one or more metals of the group columbium, tantalum and vanadium;

(b) one or both of the metals molybdenum and tungsten;

and

(c) titanium and/or zirconium,

in certain percentages by weight as is therein set forth.

As will be apparent to those skilled in this particular art the composition and properties of the materials disclosed in said referenced patents and patent applications are substantially different from those which are disclosed and claimed herein.

Accordingly, a principal object of our invention is to provide novel nitrided titanium alloys characterized by excellent wear and abrasion resistance.

This and others objects, features and advantages of our invention will become evident from the following detailed disclosure thereof.

SUMMARY OF THE INVENTION We have discovered that by the specific alloying as taught herein, prior to nitriding, the brittleness and chipping exhibited by previous titanium alloys can be avoided. As shown subsequently, the alloying elements present in typical commercial titanium alloys do not produce the same improvement and the nitrided commercial titanium alloys show chipping similar to nitrided titanium.

The scope of applicability of the present alloys is different from that disclosed in the above identified pending U.S. patent applications and Belgium patents. The present alloys have a utility for wear and abrasion resistance and less severe cutting applications. The principal advantage of these alloys is the greater ease of melting and fabrication of the unnitrided titanium base alloys as compared to the previous compositions containing higher amounts of columbium, tantalum or vanadium. This advantage will lead to their usage in applications requiring simple or complex shapes readily fabricated prior to nitriding.

DESCRIPTION OF THE PREFERRED EMBODIMENT In our experimental work a series of alloys were melted under an inert atmosphere in a non-consumable electrode arc furnace using a water-cooled, copper hearth. High purity materials (greater than 99.5%) were used for the alloy charges and generally weighed about 50 grams. These procedures are of course quite well known to those skilled in the art.

The alloys were cut into specimens approximately A; inch thick and reacted in nitrogen at atmospheric pressure. The resulting thickness and microhardnesses of the various reaction zone or layers were determined using standard metallographic techniques. Several tests were used to evaluate the strength and toughness of these materials for potential use in abrasive wear or metal cutting applications.

The metal cutting tests were performed using the nitrided materials as tool inserts x x inch having a 0.030 inch nose radius which was used as a section of the cutting surface. Such radii were ground on the specimens prior to nitriding.

The alloy samples thus prepared were subsequently nitrided. For nitriding we used a cold wall furnace employing a molybdenum heating element and radiation shields with the furnace being evacuated to five microns pressure and flushed with nitrogen prior to heating. Temperatures were measured with an optical pyrometer, namely a Leeds and Northrop Optical Pyrometer, catalog number 862, sighting on an unnitrided molybdenum heating element which completely surrounded the specimens. The temperatures given herein are corrected from this source. We used a correction factor determined by using a tungsten-rhenium thermocouple in conjunction with the sightings of the aforesaid optical pyrometer.

Following nitrided sample preparation, lathe turning tests were run on A181 4340 steel having a hardness of Rockwell C (Re), 44. A feed rate of 0.05 inch per revolution and depth of cut of 0.050 inch were used. A standard negative rake tool holder was employed with a back rake and a 15 side cutting edge angle.

Our principal citerion in determining whether the present nitrided materials passed or failed and thus whether they are useful or not for purposes of the present invention was the ability to remove two cubic inches of hardened steel at a speed of 200 s.f.m. (surface feet per minute). Furthermore, we evaluated the toughness and chipping resistance by using a conical diamond hardness indentation (standard Rockwell A scale-60 kg. load) and evaluating whether chipping occurred around the hardness impression using a X eyepiece magnifier. All of our materials pass the 200 s.f.m. cutting test and do not exhibit chipping around the Ra hardness indentations. Of course our nitrided materials have substantial applicability in wear and abrasion applications other than cutting, but the cutting test is a standardized and readily reproducible test.

Limited erosion data were also obtained using an adapted S. S. White Airabrasion unit. This involves a stream of argon gas used as a propellant to direct alumina particles through a 0.018 inch I.D. nozzle. The test surface is 0.4 inch from the nozzle and the gas fiow is perpendicular to the specimen. The abrasive Powder No. 1, described as a fine crystalline grade of aluminum oxide with an average particle size of 27 microns.

We have discovered a novel group of alloys capable of being nitrided and which are then characterized by high surface hardness and useful wear resistance. These materials are formed when alloys within our prescribed compositional ranges as hereinafter taught are reacted with nitrogen or an environment which is nitriding to the alloys at elevated temperatures. The hardening reaction is typical of the internal oxidation or nitridation techniques well known in the art. The volume of hard constituents formed at the surface is high in our materials. The unnitrided compositions within the scope of our invention are solid solutions and as nitrogen dilfuses inwardly, there is developed a variety of nitrided phases. Nitride formation decreases inwardly from the surface and this grading contributes to the excellent thermal and mechanical shock resistance of the material. Our materials are characterized by being graded with the degree of nitride formation lessening as one moves inward from the surface.

Unalloyed titanium was nitrided for 2 hours at 2250" F. and 2850 F. Both specimens showed chipping at the edges as a result of cooling from the nitriding temperature and Ra hardness indentations resulted in serious chipping all around the impressions. These materials fail immediately in the 200 s.f.m. cutting test.

Certain test results of the present invention are given in Table I. All of this could be written as a series of examples (and should be considered as such) but for the purpose of brevity we present the data in tabular form.

TABLE I Nitriding treatment Tempera- Cutting ture Time test Composition (weight percent) F.) (hr.) results Unalloyed TL 2,250 2 F Do 2,850 2 F Ti-l-GAAV 2,850 2 F Ti-3Al-13V-11Gr 2,850 2 F Ti-20V 2,450 4 F 2,450 4 F 2,650 2 F 2,850 2 F 2,850 4 P 2,850 4 P 2,450 2 P 2,250 8 P 2,850 2 F 2,850 2 F 3,050 2 F 3,050 2 F 3,050 2 F 3,250 2 P 3,250 2 P 3,250 2 F 3,250 2 P 3,250 2 P 2,850 2 F Ti-30Ta 3,050 2 F Ti-30Ta 3,250 4 F Ti'30Ta-10Mo 3,250 2 P Ti-30Ta-20Mo 3,050 2 P Ti-30Ta-20Mo. 3,250 2 P Ti-50Ta-10Mo 3,250 2 P 'Ii-30Ta-20W 3,050 2 P 'li-50'Ia-l0W 3,250 2 P T1-45Ta-I5W 3,250 2 P 'li-20Ta-40W-. 2,850 2 P NoTE.P=Pass, removes 2 en. in. of 4,340 steel workpiece at 200 s.f.m:

F=Fails at 200 s.f.m.

The possibility that the alloying elements present in commercial titanium alloys might result in an improved nitrided material was examined. Two of the more highly alloyed commercial materials, Ti-6Al-4V and Ti-3Al-l3V- llCr were nitrided at 2850 F. for 2 hours. Chipping of the edges occurred during cooling and metallographic examination confirmed that similar to unalloyed titanium, thick continuous nitrided surface layers existed. The Ti- 6Al-4V alloy exhibited a 0.010 inch layer and the Ti-3Al-13V-11Cr alloy showed a 0.002 to 0.005 inch layer with a serrated boundary between the nitrided layer and the substrate. Both of these materials failed immediately in cutting at 200 s.f.m

The nitrided binary alloys containing 20 or 40% vanadium fail the cutting test and show pronounced chipping around Ra indentations. The nitrided titanium-rich alloy Ti-10V-10Mo also does not pass the test criteria; extensive spalling of the surface was noted when Ra impression was attempted. However, the three nitrided T1-V-Mo compositions shown in Table I, Ti-30V-20Mo, Ti-30V- 15Mo, and Ti-40V-l0Mo pass the 200 s.f.m. cutting test and show no chipping around the Ra impressions. Rapid cooling from the nitriding temperature did not produce any spalling even at sharp corners. Thus one group of materials which meet the above described criteria consist essentially of:

from 15% to 47% vanadium;

from 8% to 40% molybdenum or tungsten;

from 45% to 68% titanium; with the ratio of vanadium to titanium being greater than 1 3.

The nitrided i-Cb binary alloys shown in Table I fail to pass the test criteria. The nitrided alloy Ti-10Cb-10Mo fails the cutting test and exhibits chipping around Ra indentations. However, at higher columbium contents (Table I), the nitrided Ti-Cb-Mo alloys pass the test criteria. Similar behavior is noted with tungsten additions; nitrided Ti-30Cb-20W and Ti-20Cb-40W pass.

Fabricability of a number of unnitrided Cb-Ti-Mo and other alloys was evaluated. Compositions such as Ti- 3OCb-10Mo and Ti-25Cb-15Mo were readily cold rolled at room temperature to reductions of or more.

At high molybdenum or tungsten levels, the unnitrided alloys become brittle and difiicult to fabricate. For example, a small arc-cast ingot of Ti-Cb-65Mo broke into many pieces during surface grinding. A cutter specimen was carefully machined from the largest broken piece and after nitn'ding at 3250" F. for 2 hours, it failed to cut a 200 s.f.m. The Ra hardness indentation did not chip. Since molybdenum and tungsten do not form stable nitrides at the nitriding temperatures, less reaction and hardening is produced with the high molybdenum alloy: Thus such materials are not within the scope of our invention.

The preferred columbium containing nitrided alloys consist essentially of:

from 13% to 46% columbium;

from 8 to 50% molybdenum or tungsten;

from to 68% titanium; with the ratio of columbium to titanium being greater than l/3 but less than 1.

Another nitrided alloy system included herein is Ti-Ta- Mo and/or W. Similar alloying trends were noted. Nitrided Ti-20Ta and Ti-Ta did not cut at 200 s.f.m., Ra hardness indentations showed serious chipping, and hence these materials are not acceptable.

Unnitrided alloys such as Ti-30Ta-20Mo were readily cold fabricated; this composition was directly rolled at room temperature from an arc-cast ingot about 3/8 inch thick to 0.030 inch sheet. At the higher tantalum levels the alloys remained fabricable but did not exhibit the exceptional cold formability. Three examples of nitrided Ti-Ta- Mo and four Ti-Ta-W alloys falling within our prescribed ranges are given in Table I. In every case, the nitrided alloys pass the 200 s.f.m. cutting test and do not exhibit chipping around Ra impressions.

The tantalum containing nitrided alloys in this system falling within the following composition range are included within the scope hereof:

from 13% to 57% tantalum;

from 8 %to 5 0% molybdenum or tungsten;

from to 68% titanium; with the ratio of tantalum to titanium being greater than 1/3.

All of the six elemental components of our invention readily interalloy to form simple solid solutions. The nitriding behavior is similar for the alloys containing vanadium, columbium, or tantalum except that the vanadium containing alloys can be nitrided at a lower temperature. Accordingly, the preferred nitrided materials can contain from 3 to 6 components if at least one of the group vanadium, columbium, or tantalum, and at least one of the group molybdenum or tungsten is present in addition to titanium. When 4 to 6 metallic components are present in the nitrided alloys the allowable compositional ranges are defined by the formulae set forth below; these formulae also define the limitations in a ternary system. Such formulae represent linear proportionate amounts based on weight percentages.

A modest mathematical statement is required. In the present disclosure and claims the following ratios shall have the following meanings:

(That is, the concentration in Weight percent of vanadium to total vanadium, columbium and tantalum.) Similarly In making the various alloyed, nitrided systems of this invention-that is, alloys consisting essentially of titanium; at least one metal of the group consisting of vanadium, columbium, and tantalum; and at least one metal selected from the group consisting of molybdenum and tungsten the following composition requirements must be met for the nitrided materials:

(1) The minimum combined molybdenum and tungsten content is 8% (2) The maximum combined molybdenum and tung sten content is given by (3) The maximum titanium content is given by the ratio:

(4) The minimum titanium content is given by:

[46' /2(Mo+W-8)]B+35C+45A A variety of nitrogen containing environments can be used to produce similar hardened materials. However, upon reacting in a much lower nitrogen potential environment, the effect of lowered nitrogen availability is observed and a somewhat modified reaction product may be obtained. Sincev our surface hardened materials are in a thermodynamically metastable condition, a variety of heat treatments, including multiple and sequential treatments, can be used to modify the reaction product and resulting properties whether performed as part of the overall nitriding reaction or as separate treatments. The materials can also be nitrided at higher temperatures (and times) that normally would produce some embrittle ment and then subsequently annealed in inert gas as a tempering or drawing operation to improve toughness. This duplex treatment results in a deeper reaction product with the hardness-toughness relationship controlled by the tempering temperature and time. The annealing treatment aforesaid may also be carried out in partial pressures of nitrogen.

We have modified our nitrided material by combining nitriding with oxidizing or boronizing. However, the amount of reaction with the other hardening agents must be limited, a majority of the weight pick-up is due to nitriding, and these are essentially nitrided materials. For example, we have found that the Ti-30Cb-10Mo alloy may be reacted at lower temperatures in a combined oxidizing and nitriding environment. We have also preoxidized at a temperature where little reaction would occur with nitrogen alone and then subsequently nitrided. In another example, Ti-30Cb-l0Mo was nitrided at 3250 F. for 2 hours and subsequently boronized at 2650 F. for 4 hours. The structural features of such a material are very similar to the alloy only nitrided; the hardness grades inwardly and of the total Weight pick-up, over is due to nitriding. A smooth surface layer about 0.5 mil thick forms due to the boronizing treatment that is much harder than the nitrided surface. Up to 25% of the nitrogen pick-up can be replaced by oxygen and/or boron.

The present useful alloys could be produced by powder processing techniques. Also such alloys could be employed on another metal or alloy as a surface coating or cladding and with the proper selection, a highly ductile or essentially unreacted substrate can be obtained. The nitrided material can be used as a mechanically locked insert or it can be bonded or joined by brazing, for example, to a substrate.

Spraying and/or fusing the desired alloy onto the surface are among the various methods for depositing the ternary or more complex (Cb, Ta, V)-Ti-(Mo, W) alloys. Small other additions may be made to our alloys to enhance the coatability. A variety of direct deposition methods may be employed or alternate layers could be deposited followed by a diffusion annealing treatment. Another surface alloying procedure involves titanizing-for example, a process whereby titanium is dilfused into a Cb-W alloy substrate.

The high surface hardness of the nitrided alloys has been confirmed by 50 and 200 gram diamond pyramid TABLE II Mierohardness (DPH) at distance Nitrlding from suriaee (mile) of- Composltion, tempera- Weight percent ture F.) 0. 1 2 4 8 'Ii-lOCb-IOMO 3, 050 2, 060 1, 560 1, 890 400 420 Tl-30Cb-20W 3, 250 2, 260 2, 370 1, 460 630 530 Tl-lOV-IOMO 2, 850 2, 260 2, 48 410 400 400 Ti-30V-2Mo 2, 850 1, 310 1, 160 700 390 4400 Although not part of our test criteria, some comparative erosion data were obtained and are shown in Table III. The erosion number was calculated by dividing the erosion time in seconds by the erosion pit depth in mils. Data are given for nitrided unalloyed titanium and five representative nitrided alloys falling within our invention. The lower Wear rate as evidenced by a higher erosion number is apparent for our nitrided alloys.

TABLE III nitriding Erosion treatment 1 number Composition (weight percent) F.) (seconds/mil) Unalloyed Ti--- 2, 850 13 Tl-30Cb-1OMO 3, 250 36 Ti-30Cb-20W 3, 250 40 Ti-2OCb-40W- 3, 250 48 Ti45Ta-15W 3, 250 80 Ti-30Ta-20W 3, 250 53 1 Nitriding time 2 hours.

The nitrogen pick-up is in excess of 1 mg. per sq. cm. for all of the examples shown in Table 1. However, the amount of nitrogen required for an equivalent surface hardness is substantially reduced when the material is used as a thin blade edge or sheet or as a thin coating or cladding. Also, such materials may be used for a wide variety of applications requiring wear and abrasion resistance where the requirement for surface hardness may be less than that required for metal cutting. Thus, for certain applications, the nitrogen pick-up might be 0.1 to l mg./sq. cm. of surface area.

We have also observed the excellent corrosion resistance of both the alloys and the nitrided alloys in strong acids, and these materials could effectively be employed for applications requiring both corrosion and abrasion resistance. Both the alloys and the nitrided alloys possess good structural strength. Thus, the materials can be employed for applications involving wear resistance and structural properties (hardness, strength, stiEness, toughness) at room and elevated temperatures. Other useful properties of the nitrided materials include good electrical and thermal conductivity, high melting temperature, and thermal shock resistance.

Although the alloys receptive to nitriding can be produced by coating or surface alloying techniques, many uses involve the forming and machining of a homogeneous alloy. One of the advantages in utility of these materials is our ability to form the metallic alloys by cold or hot working and/or to machine (or hone) to shape in the relatively soft condition prior to final nitriding. Only minimal distortion occurs during nitriding and replication of the starting shape and surface finish is excellent. The final surface is reproducible and is controlled by original surface condition, alloy composition, and nitriding treatment. For some applications, the utility would be enhanced by lapping, polishing, or other finishing operations after nitriding. The nitrided surface is quite hard but only a small amount of material removal is required to produce a highly finished surface.

It will be understood that various modifications and variations may be affected without departing from the spirit or scope of the novel concepts of our invention.

We claim as our invention:

1. A graded, nitrided ternary or higher alloyed material consisting essentially of:

(a) titanium;

(b) at least one metal from Group I consisting of vanadium (V), columbium (Cb) and tantalum (c) at least one metal selected from Group 11 consisting of molybdenum (Mo) and tungsten (W);

(d) the nitrogen weight pick-up is at least 0.1 milligram per square centimeter of surface area;

(f) the minimum Group II (content is 8% and the maximum Group 11 content is defined by the expression 40A +50B+5OC;

(g) the minimum titanium content is defined by the expression [46 A (Mo+W8)]B+35C+45A and the maximum titanium content is defined by the expression Ti Group I and (h) wherein the nitriding extends to at least a depth of 0.0005 inch. 2. The material defined in claim 1 wherein the surface microhardness is at least 800 diamond pyramid numerals and the reaction depth to which such hardness is developed is at least .0005 inch.

3. The material defined in claim 1 wherein: (a) the content of vanadium is 15% to 47%; (b) the content of at least one metal selected from the group consisting of molybdenum and tungsten is 8% to 40%;

(c) the content of titanium is 45% to 68%; with (d) the ratio of vanadium to titanium being greater than 1/3. 4. The material defined in claim 1 wherein: (a) the content of columbium is 13% to 46%; (b) the content of at least one metal selected from the group consisting of molybdenum and tungsten is 8% to 50%;

(c) the content of titanium is 25% to 68%; with (d) the ratio of columbium to titanium is greater than 1/ 3 but less than 1.

5. The material defined in claim 1 wherein:

(a) the content of tantalum is 13% to 57%;

(b) the content of at least one metal selected from the group consisting of molybdenum and tungsten is 8% to 50%;

(c) the content of titanium is 35% to 6 8%; with (d) the ratio of tantalum to titanium being greater than U3.

6. The material defined in claim 1 wherein up to 25% of the nitrogen weight pick-up is replaced by a material selected from the group consisting of oxygen and boron.

meter.

7. The material as defined in claim 1 wherein the nitrogen pick-up is at least 1 milligram per square centi- References Cited UNITED STATES PATENTS Wyatt et a1. 148133 X Bomberger 75174 Lenning et a1 75-174 X Douglass et al. 75174 X Wood 14831.5 X

10 OTHER REFERENCES US. Cl. X.R. 29182.2, 182.5; 75-134, 174, 175.5, 177; 117l30; 148-20.3, 32, 34

d PATENT N0.

DATED |NV ENTOR(S) Column 3,

Column 3,

Column 3,

Column 4,

"a Column 4,

Column 4,

Column 5,

Column 7,

Column 8,

[SEAL] UNITED STATES PATENT AND TRADEMARK OFFICE CETIFICATE OF CORRECTION 3,677,832 July 18, 1972 1 Ray J. Van Thyne and John J. Rausch It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 58, "nitride" should read nitrided line 16, '0. 05" should read 0. 005

line 20, "citerion" should read criterion line 42, after "abrasive" insert is Airabrasive Table l, the third alloy should read Ti-6Al-4V Table 1, line l3, "10 No" should read 10 Mo line 63, "i" should read Ti line 4, "a" should read at";

Table II, "Ti-SOV-ZMo" should read Ti-3OV-2OMo line 32, strike the parenthesis Twenty-second D3) of February 1977 A ttest:

RUTH C. MASON Arresting Officer C. MARSHALL DANN Commissioner of Parents and Trademarks 

